Explaining the other comment: 项is a quantifier, doesn’t really have the exact meaning. For example, a DROP of water/ a BUNCH of flower. Like many other phrases, 一条蛇、一头大象、一桶水、一张纸，they are all quantifiers. Just don’t worry about the understanding of it, it’s a quantifier and has a vague meaning. The only thing you should do is to remember it instead of understand the meaning, the quantifiers in Chinese all have close meanings but used in describe different phrases. When first begin to learn Chinese, ALL the Chinese children have the same problem as you, they have to remember all the collocation of phrases. When every Chinese was little， we struggled to remember the collocation of phrases and all the children have to fill in the blanks like： 一（ ）虫子、一（ ）衣服、一（ ）裙子…… So just remember the collocations! Hope this answer could help you！❤️

我在课本上看到了一项工作/运动/计划

作为量词使用，用来计量可以分项的事物。例如制度、条令、法规、协议、主张等。 一项制度、一项政策、一项原则、一项声明、一项指示等。

“项”是一个量词，没有什么实际意思。中文里有很多量词，比如个、只、本、座、张。形容不同的东西，要用不用的量词，比如，一个人，一本书，一座房子，一张桌子。记住这些搭配就可以了。